The manufacture of microelectronic systems calls for joining together large numbers of chips with quite a few (on the order of several dozens) external leads. This problem is further aggravated by the fact that the number of external leads employed in integrated circuits has been growing in recent years, with this process being expected to continue until single-chip microelectronic systems gain wide acceptance. It is doubtful, however, that such acceptance can be gained in the nearest future, and the need to join together multi-lead instruments will be growing. In order to efficiently solve this problem, a circuit panel with at least two levels of wiring needs to be developed.
Of key importance to any process for manufacturing circuit panels with multi-level wiring is the technique of manufacturing crossovers, i.e., wiring conductors which intersect in the overall wiring structure without being connected with each other, and contacts between individual wiring levels.
Most extensively used at present are processes for manufacturing circuit panels, enabling one to obtain conductors at different levels of wiring separated by means of dielectric at crossovers, said conductors contacting, where necessary, via holes in dielectric that are filled partly or fully with a conducting medium.
Such processes include processes for manufacturing double sided and multi-layer printed-circuit boards with metallized holes, thick-film and multi-layer ceramic techniques, as well as newly developed processes for manufacturing multi-layer bases of thin layers of organic dielectric pressed on a metal substrate characterized by a low coefficient of thermal expansion (cf., Ch.L. Lassen, Wanted: a New Interconnection Technology Electronics, vol. 52, No. 20, 1979, pp. 113-121).
Common to all of the afore-listed processes is the technique of successively building up layers on a dielectric base in which holes are made in one way or another and said holes are filled with conducting medium. Therefore, a large number (up to several thousand) of holes need to be provided in the board structure in required spots and said holes are to be metallized in some manner for providing therein electrical contacts between different levels of wiring. This stage of the manufacturing process is the most labor-consuming one and causes the most rejects in manufacture and failures in the course of board operation, due to difficulties involved in ensuring a high reliability of interlevel contacts. Even in the case of progressive drilling techniques such as laser drilling, this operation remains most complicated, calls for a high accuracy of blank positioning and requires the use of costly equipment.
While it is sometimes possible to make holes and metallize them in groups, as in multilayer ceramic structures or in the case of using the techniques of mask printing of conducting the dielectric layers, the resulting conductors prove inadequate for some applications, there is observed a loss of accuracy of board dimensions and relative position of contact pads, as well as an increase of electrical capacitances and interconnections between conductors in the wiring (cf., M. L. Topfer, Thick-Film Microelectronics, Van Nostrand Reinhold Company, New york, 1971).
Even though circuit panels with beam jumpers on metal supports satisfy a wide range of requirements placed upon hybrid integrated circuits and microelectronic systems of other types, owing to their mechanical and electrical properties, crossovers of this type can presently find no extensive application. This is due to the peculiarities of the existing technology of manufacturing beam jumpers, which involves twenty production steps (cf., C. E. Iowett, The Engineering of Microelectronic Thin and Thick Films, London, 1976).
Such a complexity and labor-consuming nature of this process are due to the fact that the height of crossovers depends upon the auxiliary titanium-copper-titanium layer obtained by sputtering and electro-plating, applied on top of the main level of wiring and fully removed later on by etching. Holes for supports are formed in the auxiliary layer by photolithography and etching, after which a port for crossover beam is formed in a layer of photoresist and the crossover proper is formed by electroplating of gold. This technology is only suitable for manufacturing small-size boards on rigid bases, this restricting the scope of its possible application.
As regards the manufacture of large-area circuit panels, better possibilities are offered by a metal substrate inasmuch as metal is totally free of shrinkage which is to some degree inherent in flexible dielectric of all types.
There is known in the art a process for manufacturing panels for hybrid integrated circuits, which involves the use of a metal substrate (cf., Japanese Patent No. 52-551, class 99(5) C 21, of 1977). In said process, film conductors and circuit elements are formed by conventional means on one side of a copper or iron substrate and interconnection components are assembled, whereupon this side of the substrate is potted with epoxy resin. Then, photolithography and etching are performed over the backside of the substrate to form support elements which are further used as leads.
Said prior art process is primarily disadvantageous in that the manufacture of crossovers is complicated and labor-consuming, because the process as such only provides a single level of wiring and offers no solution to the problem of making crossovers without resorting to the afore-described conventional techniques.
Further known in the art is a process for manufacturing a lead frame for an electronic instrument, which is a particular case of circuit panel manufacture (cf., Japanese Patent No. 48-36111, class 99(5) C 21, of 1974). According to said process, on a substrate there are formed conductors of required shape from a metal that is selectively etchable with respect to the metal of the conductors, the board base is laminated on one side of the substrate, and the latter is etched over the entire thickness thereof selectively with respect to the conductors.
Said process further comprises applying masking layers lying opposite expanded portions of the conductors onto the metal substrate on the side opposite from the conductors, after which the etching reagent etches everything, but the conductors and metal substrate portions protected by masking layers. As a result, the support elements formed by the metal substrate are separated from each other and connected to narrow portions of the conductors formed of deposited metal.
Said prior art process, like the former one, suffers from the complexity and labor-consuming nature of the manufacture of crossovers and, accordingly, of boards required to develop modern microelectronic systems.
Therefore, prior art processes appear to be primarily aimed at solving one of the two problems involved in the development of circuit panels for use in microelectronic systems: developing a large-area panel featuring a low density of interconnections in the wiring (some or other variety of printed circuit board), or developing a panel featuring a small area but a high density of interconnections in the wiring (some or other variety of microcircuit). Consequently, circuit panels of at least two types are widely used in microelectronic systems characterized by high degree of structural complexity, i.e., integrated circuits are assembled in microcircuits which are mounted, together with auxiliary elements, on a printed circuit board. In so doing, one cannot but pass through two stages in the manufacture of microelectronic systems and employ the manufacturing technology of two different types, namely, the technology of printed circuit board manufacture and hybrid-film technology, which is a serious disadvantage resulting from the present state of art with regard to interconnection techniques.